HTTP Communication

HTTP communication is a core part of modern Java applications. It allows your program to interact with web servers, APIs, and external services using standard web protocols.

In this guide, you will learn how HTTP works in Java and how to send GET and POST requests step by step.

What is HTTP Protocol?

HTTP (HyperText Transfer Protocol) is a communication protocol used to transfer data between a client and a server over the internet.

In simple words: HTTP is the way a browser or application communicates with a web server.

Example: When you open a website, your browser sends an HTTP request and the server returns a response (web page data).

How HTTP Works

Basic flow:

Client (Java App / Browser) → Sends Request → Server

Server → Processes Request → Sends Response → Client

Common HTTP Methods

• GET → Retrieve data from server
• POST → Send data to server
• PUT → Update existing data
• DELETE → Remove data from server

👉 These methods define the type of operation a client wants to perform on the server.

URL Class in Java

The URL class is used to represent a web address.

Example

import java.net.URL;

public class URLExample {
    public static void main(String[] args) throws Exception {

        URL url = new URL("<https://jsonplaceholder.typicode.com/posts>");

        System.out.println("Protocol: " + url.getProtocol());
        System.out.println("Host: " + url.getHost());
        System.out.println("Path: " + url.getPath());
    }
}

Explanation

• getProtocol() → Returns protocol (e.g., https)
• getHost() → Returns domain name (e.g., jsonplaceholder.typicode.com)
• getPath() → Returns endpoint path (e.g., /posts)

HttpURLConnection in Java

HttpURLConnection is used to send HTTP requests and receive responses in Java. It supports GET, POST, and other HTTP methods.

In simple words: It acts as a bridge between your Java application and a web server, allowing you to send requests and get responses.

• It is part of java.net package
• Works with URL to establish connection
• Allows setting request type, headers, and reading response data

👉 Commonly used for calling APIs and communicating with web services

Making a GET Request

Example

import java.net.*;
import java.io.*;

public class GetRequestExample {
    public static void main(String[] args) throws Exception {

        URL url = new URL("<https://jsonplaceholder.typicode.com/posts/1>");

        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setRequestMethod("GET");

        BufferedReader br = new BufferedReader(
                new InputStreamReader(con.getInputStream())
        );

        String line;
        StringBuilder response = new StringBuilder();

        while ((line = br.readLine()) != null) {
            response.append(line);
        }

        br.close();

        System.out.println(response.toString());
    }
}

Explanation

• openConnection() → Opens HTTP connection to the URL
• setRequestMethod("GET") → Sets request type as GET
• getInputStream() → Reads response from server
• BufferedReader → Reads response line by line
• StringBuilder → Stores complete response

Making a POST Request

Example

import java.net.*;
import java.io.*;

public class PostRequestExample {
    public static void main(String[] args) throws Exception {

        URL url = new URL("<https://jsonplaceholder.typicode.com/posts>");

        HttpURLConnection con = (HttpURLConnection) url.openConnection();

        con.setRequestMethod("POST");
        con.setDoOutput(true);

        String jsonInput = "{\\"title\\":\\"Java\\",\\"body\\":\\"HTTP Post\\",\\"userId\\":1}";

        OutputStream os = con.getOutputStream();
        os.write(jsonInput.getBytes());
        os.flush();
        os.close();

        BufferedReader br = new BufferedReader(
                new InputStreamReader(con.getInputStream())
        );

        String line;
        StringBuilder response = new StringBuilder();

        while ((line = br.readLine()) != null) {
            response.append(line);
        }

        br.close();

        System.out.println(response.toString());
    }
}

Explanation

• setDoOutput(true) → Enables sending data in request body
• getOutputStream() → Sends data to server
• JSON string → Represents data sent in POST request
• Response is read same as GET request

Handling HTTP Response

Important Methods

• getResponseCode() → Returns HTTP status code (200, 404, etc.)
• getInputStream() → Used for successful response
• getErrorStream() → Used for error response

Example

int status = con.getResponseCode();

if (status == 200) {
    System.out.println("Success");
} else {
    System.out.println("Error: " + status);
}

Common HTTP Status Codes

• 200 → OK (request successful)
• 201 → Created (resource successfully created)
• 400 → Bad Request (invalid request from client)
• 404 → Not Found (requested resource not found)
• 500 → Server Error (issue on server side)

Real-World Use Cases

HTTP communication is used in:

• REST APIs (Spring Boot applications)
• Payment gateways (processing online transactions)
• Third-party integrations (connecting external services like APIs)
• Mobile and web backends (client-server communication)
• Data fetching from external services (APIs, microservices)

HTTP is the backbone of web communication between applications and servers.

Advantages

• Simple and widely used protocol
• Works with almost all web services and APIs
• Supports multiple data formats (JSON, XML)
• Platform independent (works across different systems and languages)

Limitations

• HttpURLConnection is low-level and verbose (requires more code)
• Requires manual handling of input/output streams
• Not ideal for large-scale or complex applications

Modern alternatives:

• HttpClient (Java 11+)
• Libraries like OkHttp, Retrofit

Common Mistakes to Avoid

• Not closing streams properly
• Ignoring HTTP response codes
• Not setting the correct request method (GET/POST)
• Sending invalid JSON format in request body
• Forgetting setDoOutput(true) while making POST requests

Conclusion

HTTP communication is essential for building modern Java applications.

• Uses URL and HttpURLConnection for communication
• Supports GET and POST requests
• Enables interaction with APIs and web services

👉 HTTP allows Java applications to communicate with web servers and exchange data over the internet.